1. Field of the Invention
The present invention generally relates to phase detection using sampled data, and more particularly to a clock adjustment device employed in a data reproduction apparatus such as an optical disk unit or a magnetic disk unit for adjusting the phase of a clock signal used for reproducing recorded data based on values sampled in synchronism with the clock signal from a reproduced signal from a recording medium and a data reproduction apparatus including such a clock adjustment device. The present invention also relates to an edge detection device and method for detecting an edge of the reproduced signal.
2. Description of the Related Art
Recently, as information is being recorded on a recording medium such as a magneto-optical disk at a higher density, more efforts are being made to increase a data transfer rate in the reproduction system of the magneto-optical disk. As a method of performing high-speed information reproduction with high accuracy from such a magneto-optical disk recorded with information at high density, a recording and reproduction method according to PRML (Partial Response Maximum Likelihood) is proposed.
According to this PRML recording and reproduction method, a magneto-optical disk is recorded with a signal obtained by modulating data to be recorded into a partial response (PR) waveform. Then, a reproduced signal from the magneto-optical disk is sampled in synchronism with a predetermined clock signal so as to obtain sampled values. Data having maximum likelihood is reproduced from the sampled values in accordance with a maximum likelihood (ML) algorithm such as the Viterbi decoding algorithm.
In a data reproduction apparatus based on the above-described PRML recording and reproduction method, such as an optical disk unit, it is necessary to adjust the phase of a clock signal so that sampled values obtained by sampling a reproduced signal from a recording medium such as a magneto-optical disk in synchronism with the clock signal are characterized by a partial response waveform. For instance, in the case of a PR(1,1) waveform, the sampled values are 1s (peak), 0s (center), and xe2x88x921s (bottom).
A conventional clock adjustment device binarizes a reproduced signal using a predetermined slice level and adjusts the phase of a clock signal using a PLL circuit so that the clock signal synchronizes with the rising or trailing edges of the binarized reproduced signal.
Further, instead of performing analog processing on the reproduced signal as described above, it is practicable to detect an edge part of the reproduced signal based on the state of change of sampled values obtained by sampling the reproduced signal in synchronism with the clock signal. For instance, if first through third successive sampled values are obtained from a reproduced signal of a given pattern so that the reproduced signal monotonously increases in level between the first and third sampled values with the second or third sampled value being higher than a reference level, it may be determined that the three sampled values correspond to a rising edge part of the reproduced signal.
On the basis of relationships among three sampled values that, correctly, should be obtained from the reproduced signal of the given pattern, the phase difference of the clock signal is obtainable based on relationships among the above-described three successive sampled values corresponding to the rising edge of the reproduced signal. The phase of the clock signal is adjustable so as to eliminate the phase difference.
In a clock adjustment device that detects edges of a reproduced signal based on the state of change of sampled values obtained in synchronism with a clock signal from the reproduced signal and adjusts the phase of the clock signal so that the clock signal synchronizes with the edges as previously described, edge detection is not stably performed if the clock signal is in a step-out state, or in a state where the phase of the clock signal is unlocked, in process of phase adjustment.
Suppose that a densest pattern (hereinafter referred to as a 2T pattern Q2T) of a PR(1,1) waveform as shown in FIG. 1 is written to a magneto-optical disk as a synchronization pattern (a given pattern) and the phase of the clock signal is adjusted based on a reproduced signal of the 2T pattern Q2T. In such a case, with the clock signal being in a step-out state with the phase of the clock signal being unmatched with the correct sampling timing of the 2T pattern Q2T, of successive sampled values Ytxe2x88x926, Ytxe2x88x925, and Ytxe2x88x924, the sampled value Ytxe2x88x925, for instance, may be recognized as a rising edge of the reproduced signal, or of successive sampled values Ytxe2x88x923, Ytxe2x88x922, and Ytxe2x88x921 also having an increase tendency (Ytxe2x88x923 less than Ytxe2x88x922 less than Ytxe2x88x921), the sampled value Ytxe2x88x922 may be recognized as a rising edge.
In this case, even if it is determined from the sampled value Ytxe2x88x925 that xe2x80x9cthe phase lagsxe2x80x9d, it is determined from the sampled value Ytxe2x88x922 that xe2x80x9cthe phase leadsxe2x80x9d, thus preventing the phase of the clock signal from being adjusted.
It is a general object of the present invention to provide a clock adjustment device, a data reproduction apparatus including a clock adjustment device, and an edge detection device and method in which the above-described disadvantage is eliminated.
A more specific object of the present invention is to provide a clock adjustment device for performing stable adjustment on the phase of a clock signal based on the state of change of sampled values obtained in synchronism with the clock signal from a reproduced signal corresponding to a given pattern and a data reproduction apparatus including such a clock adjustment device.
Yet another more specific object of the present invention is to provide an edge detection device and method for detecting an edge of the reproduced signal with good accuracy so that the phase of the clock signal can be adjusted earlier.
The above objects of the present invention are achieved by a clock adjustment device for a data reproduction apparatus for reproducing recorded data from a recording medium in accordance with a maximum likelihood decoding algorithm corresponding to a recording code of partial response by using sampled values obtained by sampling a reproduced signal corresponding to a predetermined synchronization pattern from the recording medium in synchronism with a clock signal, the clock adjustment device including an edge detection part detecting an edge of the reproduced signal based on a state of change of the sampled values, a timing signal output part outputting a timing signal in a given cycle determined by the synchronization pattern from a timing at which the edge of the reproduced signal is detected, and a phase adjustment part adjusting a phase of the clock signal when the edge of the reproduced signal is detected and every time the timing signal is output.
According to the above-described clock adjustment device, after the phase of a clock signal is adjusted when an edge of a reproduced signal corresponding to a predetermined synchronization pattern is detected based on the state of change of sampled values of the reproduced signal, the phase of the clock signal is adjusted in a given cycle determined by the synchronization pattern. Therefore, after the edge of the reproduced signal is detected, the phase of the clock signal is adjustable in a cycle in which the reproduced signal rises. The given cycle determined by the synchronization pattern corresponds to a cycle in which the synchronization pattern rises.
The above objects of the present invention are also achieved by a data reproduction apparatus for reproducing recorded data from a recording medium in accordance with a maximum likelihood decoding algorithm corresponding to a recording code of partial response by using sampled values obtained by sampling a reproduced signal corresponding to a predetermined synchronization pattern from the recording medium in synchronism with a clock signal, the data reproduction apparatus including a clock adjustment device which includes an edge detection part detecting an edge of the reproduced signal based on a state of change of the sampled values, a timing signal output part outputting a timing signal in a given cycle determined by the synchronization pattern from a timing at which the edge of the reproduced signal is detected, and a phase adjustment part adjusting a phase of the clock signal when the edge of the reproduced signal is detected and every time the timing signal is output.
According to the above-described data reproduction apparatus, data is reproducible from sampled values obtained from a reproduced signal corresponding to a predetermined synchronization pattern in synchronism with a clock signal whose phase is adjusted when an edge of the reproduced signal is detected based on the state of change of the sampled values and, thereafter, is adjusted in a given cycle determined by the synchronization pattern.
The above objects of the present invention are also achieved by an edge detection device for detecting an edge of a reproduced signal from a recording medium, the reproduced signal corresponding to a predetermined synchronization pattern, the edge detection device including an edge determination part determining, using first through fourth successive sampled values in an order sampled from the reproduced signal and based on a relationship between the first and third sampled values and a relationship between the second and fourth sampled values, whether or not the second or third sampled value corresponds to the edge of the reproduced signal.
According to the above-described edge detection device, edge detection is performed based on a relationship between first and third sampled values and a relationship between second and fourth sampled values of the four successive first through fourth sampled values, thereby realizing the edge detection with more accuracy compared with a conventional edge detection method that, for instance, employs three successive sampled values.
The above objects of the present invention are further achieved by a method of detecting an edge of a reproduced signal from a recording medium, the method including the steps of (a) obtaining first through fourth successive sampled values in an order sampled from the reproduced signal, (b) calculating a first difference between the first and third sampled values and a second difference between the second and fourth sampled values, and (c) determining whether the second or third sampled value corresponds to the edge of the reproduced signal based on the first and second differences obtained in the step (b).
According to the above-described method, edge detection is performed based on a difference between first and third sampled values and a difference between second and fourth sampled values of the four successive first through fourth sampled values, thereby realizing the edge detection with more accuracy compared with the conventional edge detection method employing three successive sampled values.